The present invention relates to an improved process for removing alkynes from alkene rich liquid hydrocarbon streams, such as 1,3-butadiene-rich C.sub.4 cuts, with a minimum loss of conjugated dienes present therein. In a particular aspect, the present invention relates to a process for the selective hydrogenation of alkynes present in 1,3-butadiene-rich C.sub.4 cuts obtained from steam cracking units, to produce a 1,3-butadiene rich product for use in the production of synthetic rubber.
The polymerization of 1,3-butadiene to produce synthetic rubber is an important indusrial process, with over 10 million tons being produced annually. The typical feed stocks used in the production of polybutadiene synthetic rubber contain a major proportion of 1,3-butadiene and butenes. In addition to 1,3-butadiene and butenes, these feedstocks contain significant amounts of alkynes (also called acetylenes) that must be removed prior to polymerization since alkynes poison the polymerization catalyst. The alkynes are generally removed from these feedstocks by selective hydrogenation while trying to avoid or to limit losses in 1,3-butadiene.
The process for the selective hydrogenation of alkynes must be very selective towards alkynes since highly undesirable side reactions occur. These undesirable side reactions include the polymerization and hydrogenation of olefins present in the feed such as 1,3-butadiene and butenes. Not only do these side reactions reduce the amount of desired 1,3-butadiene present in the product but they also reduce catalyst life, due to the buildup of polymer on the catalyst. Regenerations of the catalyst are possible, but are costly and induce catalyst modification that eventually leads to mechanical breakdown of the catalyst pellets which results in higher pressure drops across the bed.
It has long been known to selectively hydrogenate alkynes at high temperature in vapor phase over a copper-nickel catalyst on a SiO.sub.2 /Al.sub.2 O.sub.3 support. However, such processes have fallen to disfavor since the catalyst has to be replaced or regenerated frequently, and the loss of 1,3-butadiene and retention of alkynes is unacceptably high.
U.S. Pat. No. 4,493,906 discloses a catalyst for the removal of alkynes from liquid hydrocarbon streams. The catalyst disclosed in this patent is made of finely divided copper metal dispersed upon a well-defined gamma alumina (which may contain up to 35 wt. % alpha alumina). This gamma alumina has a surface area of 68 to 350 m.sup.2 /g with 40 to 98% of the pores having a pore diameter between 4 and 12 nanometers (nm), and 2 to 25% of the pores having a pore diameter between 100 and 1,000 nm. The support is high purity alumina, having less than 0.13 wt. % silicon and less than 0.15 wt. % sodium. This patent discloses that 0 ppm alkynes remain when a feed is treated at about 68.degree. C. with a liquid hourly space velocity (LHSV) lower than 1. However, the corresponding cycle life of the catalyst is only 51/2 days; after 6 days, about 100 ppm alkynes are detected in the effluent. It is evident that at higher values of LHSV the cycle life of the catalyst would be even shorter and/or the alkynes removal would be incomplete.
Another type of catalyst known for the selective hydrogenation of alkynes uses a Group VIII B metal in the catalyst. Palladium is the Group VIII B metal that is generally used since it is most active and selective for the hydrogenation of alkynes. However, at least 2 types of operating problems are encountered using this catalyst;
significant loss of 1,3-butadiene even at moderate conversion of alkynes; and PA1 reduced catalyst life, due to significant loss of palladium, PA1 (a) providing a 1,3-butadiene-rich C.sub.4 cut; PA1 (b) passing said cut over a palladium-based catalyst in the presence of hydrogen; PA1 (c) passing the liquid effluent from step (b) over a copper-based catalyst in the presence of hydrogen; and PA1 (d) recovering a 1,3-butadiene-rich product of reduced alkyne concentration. PA1 (a) providing a 1,3-butadiene-rich C.sub.4 cut; PA1 (b) passing said cut maintained in trickle mode over a palladium-based catalyst bed in the presence of hydrogen; PA1 (c) passing the effluent from step (b) at least partially in the liquid phase over a copper-based catalyst bed in the presence of hydrogen; PA1 (d) separating the residual hydrogen from the remainder of the effluent of step (c); and PA1 (e) recovering a 1,3-butadiene-rich product of reduced alkyne concentration. PA1 temperature: 15.degree. to 20.degree. C. (inlet) PA1 pressure: 0.5 MPa (5 bar, 0.1 MegaPascal=1 bar) PA1 LHSV: 30 1/1.h.sup.-1 PA1 H.sub.2 /alkynes molar ratio: 2:1 PA1 feed: PA1 purified effluent: PA1 cycle life: 8 to 10 months PA1 activation by overnight hydrogenation at 300.degree. to 350.degree. C., PA1 temperature: 68.degree. C. (inlet) PA1 pressure 2 to 2.5 MPa (20 to 25 bars) PA1 LHSV: 0.67 PA1 H2/alkynes molar ratio: 3:1 PA1 61 wt. 1,3-butadiene PA1 8716 ppm alkynes PA1 purified effluent: PA1 cycle length: 51/2 days PA1 total pressure: about 0.4 to 0.9 MPa, preferably about 0.6 to 0.8 MPa; PA1 hydrogen/alkynes molar ratio: about 2:1 to 20:1, preferably about 4:1 to 10:1, most preferably about 6:1; PA1 inlet temperature: adjusted with respect to the total pressure in order to maintain the feed at least partially in the liquid phase.
as clearly disclosed in Hydrocarbon Processing, March 1985, p. 52.
A new palladium catalyst is disclosed in GB No. 2,018,817. This new catalyst is not as rapidly deactivated and does not lose as much palladium as conventional catalysts, and is sometimes used with a conventional catalyst in two successive beds. However, the process using this new catalyst, even in combination with other known catalysts, does not sufficiently reduce the alkyne concentration of the effluent.
In the past few years the severity of steam cracking conditions has increased, resulting in raw C.sub.4 cuts that contain an increased concentration of alkynes, up to 1 wt. % or even higher. In addition, the demand for hydrocarbon effluents that contain even less alkynes has increased. Accordingly there is a need in the art for an improved process of removing alkynes from liquid hydrocarbon streams without significantly reducing the alkenes present therein.